Impossible Circle Optical Illusion

What's wrong with this picture?

This is another version of the Impossible 3-D Triangle. Notice how the shading works to create a believable yet impossible object. The artist’s use of different shades around the figure helps the eye track and believe this 3D optical illusion. Through this example we see the possibilities of drawing semi-realistic images of 3D objects that couldn't possibly be made in real life.